The present invention is directed to a double band press in which a continuously running sheet material is processed and comprises a rigid press frame in which reversing rollers or drums are rotatably supported in bearing supports mounted on the press frame. An endless upper press belt and an endless lower press belt are trained over different reversing drums. Within each of the press belts there is a pressure plate which faces one run of the press belt and forms, in combination with the press belt and a sliding seal, a pressure chamber for a pressure medium used in applying pressing force to the press belt. The sliding seal is positioned within a groove formed in the surface of the pressure plate so that the seal is displaceable relative to the inside surface of the associated press belt and can be pressed with a certain amount of force against the press belt. A fluid pressure means is contained within the pressure chamber bearing against the sliding seal and generating a pressing action on the press belt.
Double band presses, such as disclosed in German Offenlegungsschrift No. 24 21 296, are known for producing continuous sheet material, such as laminates, chip boards, fiber boards, electrolaminates and the like.
Elastomers resistant to approximately 200.degree. C. are used as sealing materials in sliding seals for these presses. Further, it is known in the German Pat. No. 27 22 197 to secure elastomers, pressure-setting plastics or thermoplastic seal materials in U-shaped retaining members formed of high tensile strength metal. Support angles secured in transverse bearings in the pressure plate, are fastened to the sides of the retaining members at selected distances from one another. In this arrangement friction forces are introduced into the pressure plate from the sliding seal through the retaining members and the support angles. Finally, it is known from German Pat. No 29 53 078 to arrange two sliding seals in mutually spaced relation to one another and to utilize the space between the seals for collecting pressure medium which flows out of the pressure chamber. In these sealing arrangements friction forces are introduced into the pressure plate through the retaining members and the support angles.
A common feature of all the known sealing arrangements is that the seal is formed of a relatively easily deformable material, such as an elastomer. Since the friction forces developed at the seal in the operation of the double band press far exceed the maximum allowable tensile or shearing stresses for the sealed material, the seal would be immediately destroyed if the friction forces were not introduced into the pressure plate by the retaining members and support angles. There is the disadvantage, however, that the known construction is costly in terms of design and manufacturing technology. Moreover, the pressure and temperature conditions existing in the double band press are in the upper region of the values for an elastomer sealing material. Therefore, even slight changes in these operating conditions can lead to destruction of the seal.
Another disadvantage of the organic sliding seal material used in double band presses is the poor thermal conductivity. As a result of such poor thermal conductivity, the friction heat developed by the contact of the seal with the press belt surface cannot flow into the pressure plate, rather it flows into the material to be pressed through the press belt. This characteristic leads to local linear overheating of the press belt and eventually leaves an undesirable visible stripe on the surface of the material being pressed and the stripe must be removed by trimming the border of the pressed material. As a result, there are considerable losses in material.